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I'm an engineer with experience in applied cryptography, in particular in Smart Card systems.


Nov
13
revised Strange MAC algorithm
Clearer intro
Nov
13
answered Strange MAC algorithm
Nov
13
comment Strange MAC algorithm
Somewhat related to this question. $\;$ Independently: knowing MD5(k||m) allows computing MD5(k||m||m'||m") for m' a certain known function of m and the length of k (with m' at least 9 bytes or 65 bits), and freely chosen m".
Nov
13
comment AES with weak keys
Yes. An easy statistical calculation shows that if for $j$ increasing from $0$ to $13$ we try the $128!/(128-j)!/j!$ keys with $j$ zero bits and $128-j$ one bits (using encryption of some known plaintext), we'll find a key with odds about $59.6\%$, and less than $2^{57.8}$ AES encryptions. With $j$ up to $8$, our chances to find a key are still a fair $9.7\%$, with effort less than $2^{40.5}$ encryptions. With $j$ up to $5$, $0.93\%$, with effort less than $2^{28.1}$.
Nov
13
revised What is the most computationally efficient way of generating pseudo-random permutations?
describe fastperm2
Nov
12
revised Permutation parity after cycle-walking
The probability asked is now decreasing smoothly when m decreases or n increases; avoid odds, use probability
Nov
12
revised What is the most computationally efficient way of generating pseudo-random permutations?
Fix statement on parity; polish
Nov
12
comment Permutation parity after cycle-walking
@poncho: You are absolutely right! I misinterpreted the argument given in that answer into the (incorrect): any reversible transformation leaving at least 1 bit of the state unchanged is even. What's correct is: any transformation leaving at least 1 bit of the state unchanged and without influence on the other bits is even.
Nov
12
accepted Is AES's parity key-dependent?
Nov
12
revised Permutation parity after cycle-walking
Fix what Poncho pointed
Nov
12
comment Algorithm of Big Integer class Multiplication in visual studio
@saeid: the question asking what algorithm(s) the .Net BigInt class uses is more interesting that asking "the method name". Still it is off-topic, as there is no stated relation to cryptography. I would guess that for multiplication of numbers up to few hundred bits at least, straight long multiplication is used, as it is both simple and most efficient. I would not bet for ModPow.
Nov
12
revised What is the most computationally efficient way of generating pseudo-random permutations?
Modular addition over the whole state width is good
Nov
12
revised What is the most computationally efficient way of generating pseudo-random permutations?
Add another, simpler technique to deal with parity
Nov
12
revised Permutation parity after cycle-walking
Revise motivation
Nov
12
comment Permutation parity after cycle-walking
@poncho: I think you mean addition of a key as wide as the state, which does give balanced parity. Replacing XOR with modular addition in the combination of the round function output and half state does not.
Nov
11
revised Permutation parity after cycle-walking
Add motivation
Nov
11
asked Permutation parity after cycle-walking
Nov
11
comment How to perform frequency analysis of a substitution cipher using a Base64 alphabet
[Revised] Hint: How many letters, when present as first (resp. third, fourth, sixth) position of the plaintext generate a U (resp. O R S) in first (resp. fourth, fifth, eighth) position of the Base64? How does the expected frequency of letters in plaintext translate to expect frequency in first, fourth.. (resp third, sixth..) letter of the Base64? What about digrams in third and fourth, sixth and seventh.. position in plaintext and relation to digrams in fourth and fiveth, eighth and nineth.. position in the Base64?
Nov
11
revised Can hashing become encrypting?
Discuss security proof and efficiency.
Nov
11
answered Can hashing become encrypting?